The present invention relates to an improved electromagnetic spring-wound clutch which may be advantageously employed to connect an air conditioning compressor to an engine of an automotive vehicle. This invention constitutes a novel and advantageous improvement to an ELECTROMAGNETIC SPRING-WOUND CLUTCH disclosed in copending U.S. patent application Ser. No. 866,645, filed Jan. 3, 1978, now U.S. Pat. No. 4,194,607, issued Mar. 25, 1980, which is assigned to the same assignee as this application.
An electromagnetic spring-wound clutch of the general type to which the present invention relates is disclosed in U.S. Pat. No. 3,735,847 and comprises coaxial input and output members. A clutch disc is rotatably provided on the input member. Furthermore, a coil-wound spring is wound around the input member and connected at its opposite ends to the clutch disc and output member. An electromagnetic coil attracts the clutch disc into frictional engagement with the input member, causing the coil-wound spring to be tightly wound on the input member and drivably connect the input member to the output member. When the coil is de-energized, the clutch disc disengages from the input member, and the coil-wound spring releases the input member for rotation relative to the output member.
The clutch further comprises a generally tubular cover which protectively surrounds the coil-wound spring. With the coil de-energized, the coil-wound spring radially expands due to its own force into engagement with the inner surface of the cover. Furthermore, a spring means urges the clutch disc away from the input member and into engagement with the cover when the coil is de-energized.
Several problems have become evident in this design which are overcome by the present invention. One problem is that the clutch disc, when disengaged, has a tendency to vibrate against the cover during operation of the vehicle and produce undesirable noise. The clutch disc and/or cover may even become deformed through constant vibration after prolonged use of the clutch.
Another and even more serious problem is magnetic flux leakage when the clutch disc and coil-wound spring ohmically engage the cover. Since all of these components are made of iron or steel, they constitute an integral ferromagnetic mass when ohmically engaged. The magnetic flux from the coil permeates not only the clutch disc but also the cover and coil-wound spring. Thus, the magnetic flux density in the clutch disc which is designed to attract the clutch disc to the input member is greatly reduced. It may be considered that the magnetic flux leaks from the clutch disc to the cover and coil wound spring, thereby reducing the magnetic attractive force between the coil and the clutch disc.
In order to overcome the reduced attractive force, an electromagnetic coil which produces an unnecessarily high magnetic field must be provided. Thus, the coil must be disproportionately large in size and dissipate excessive electrical current.
However, the most serious problem is that the clutch may completely fail to engage after prolonged use. Friction between the clutch disc and input member causes the clutch disc to progressively become thinner. As a result, the proportion of magnetic flux in the clutch disc becomes progressively smaller. After the clutch disc becomes so thin that the magnetic flux therein is insufficient to strongly hold the clutch disc against the input member, the clutch will no longer effectively engage and drivably connect the compressor to the engine.